1. Field of the Invention
The invention relates to a process for preparing highly pure raffinate II (a C4-hydrocarbon mixture) which has a low isobutene content and is particularly suitable for the preparation of pure 1-butene and methyl tert-butyl ether (MTBE).
2. Discussion of the Background
Isobutene-free butene mixtures are suitable for preparing highly pure 1-butene and/or for preparing butene oligomers having a low degree of branching. MTBE is a sought-after carburetor fuel component for increasing the octane number. For this purpose, there is no harm if other ethers such as methyl sec-butyl ether or oligomers of C4-olefins are present in the MTBE. High-purity MTBE which is to be used as a solvent, however, requires significantly tighter limits for the abovementioned secondary components.
MTBE and linear butenes are obtained from C4-olefin mixtures, for example the C4 fraction from steam crackers or FCC units. These mixtures include or consist essentially of butadiene, the monoolefins isobutene, 1-butene and the two 2-butenes together with the saturated hydrocarbons isobutane and n-butane. Customary work-up methods used worldwide for such C4 fractions include the following steps: first, the major part of the butadiene is removed. If butadiene can be readily marketed or there is a use for it within the company, it is separated off, for example by extraction or extractive distillation. Otherwise, it is hydrogenated selectively to linear butenes so as to leave butadiene concentrations of from 1 to 0.1%. In both cases, a hydrocarbon mixture (corresponding to raffinate I or hydrogenated crack-C4) that includes the saturated hydrocarbons (n-butane and isobutane) together with the olefins (isobutene, 1-butene and 2-butenes) remains. A possible way of removing the isobutene from this mixture is reaction with methanol to form MTBE. This leaves the saturated hydrocarbons, linear butenes and possibly a residual amount of isobutene. The C4 mixture obtained after removal of the butadiene and isobutene is referred to as raffinate II.
Depending on the further use of the two streams (MTBE and the olefin mixture, raffinate II), particular qualities of these streams are of special interest. If the isobutene from the C4 fraction is utilized for producing carburetor fuel components (be it as MTBE or oligomer), the purity of the MTBE is not subject to any critical requirements. Other ethers such as methyl sec-butyl ether and/or C4-olefin oligomers can be present in the MTBE.
In addition to the linear olefins, relatively large amounts of isobutene can be present in raffinate II if this C4 mixture is reacted, for example, over acid catalysts, to form mostly branched C4-oligomers, in particular C8- and C12-oligomers. After hydrogenation, this mixture gives a high-octane carburetor fuel component.
If the MTBE is to be used, for example, as a pure solvent or for preparing highly pure isobutene in a cleavage reaction, it is allowed to contain only small amounts of secondary components. The synthesis to form MTBE therefore has to be carried out very selectively. If the raffinate II is to be used for preparing oligomers having low iso indices, i.e. a low degree of branching, the isobutene content has to be very low, preferably less than 1000 ppm by weight. Virtually isobutene-free raffinate II is necessary if pure 1-butene is to be obtained from this raffinate II. The isobutene concentration of the raffinate II should then not exceed 450 ppm by weight. Since the boiling point difference between isobutene and 1-butene is only 0.6xc2x0 C., economical separation of the two components by distillation is not possible. In this case, isobutene has to be reacted virtually completely in the MTBE synthesis.
The highest demands are placed on the MTBE synthesis if solvent-quality MTBE is to be produced and the raffinate II is at the same time to be used for 1-butene production. Here, both a very high isobutene conversion and a very high MTBE selectivity are necessary.
The preparation of MTBE from isobutene-containing C4-hydrocarbon mixtures such as raffinate I or hydrogenated crack-C4 by reaction with methanol is frequently carried out industrially using acid ion exchange resins (sulfonic acid groups) as heterogeneous catalysts. The reaction is carried out in one or more reactors connected in series, with the catalyst preferably being present as a fixed bed. This gives a product in which methanol, isobutene and MTBE are in equilibrium. The equilibrium conversion is established in each reactor as a function of the reaction conditions (temperature, methanol excess, etc.). This means that under the reaction conditions customarily set in industrial processes, about 96% of the isobutene used is reacted. This mixture can subsequently be fractionally distilled to give a bottom fraction containing MTBE and a top fraction containing C4-hydrocarbons and methanol. After removal of the methanol present as an azeotrope, the raffinate II produced in this way is not suitable for producing pure 1-butene because of its high residual isobutene content.
To obtain virtually complete isobutene conversion, reactive distillation columns are used in industry. These are columns which contain both separation trays (or mesh packing) and catalysts on separation trays or integrated into other internals or mesh packing. In such columns, the reaction of the residual isobutene with methanol to form MTBE and the separation of the products by distillation occur simultaneously. The feed olefin mixture, for example raffinate I or selectively hydrogenated crack-C4, can also be fed into such a column. These columns are particularly useful for the abovementioned equilibrium mixture in order to achieve very high conversions. Products obtained are an azeotrope containing methanol and C4-hydrocarbons, which in the case of 1-butene production has to be virtually free of isobutene, at the top and MTBE at the bottom.
U.S. Pat. No. 4,504,687 describes a process for preparing MTBE and a low-isobutene C4 stream. Here, the reaction of a C4 stream containing both isobutene and linear butenes with methanol is carried out in a reactive distillation column in which reaction and distillation are, due to structural measures, carried out at different pressures. The division of the column in terms of pressure into a distillation section and a reaction section is structurally complicated. No information is given on the purity of the products prepared in U.S. Pat. No. 4,504,687. A large reflux ratio of 0.5-20:1 is disclosed for the reactive distillation column.
In U.S. Pat. No. 5,120,403, the same reaction is carried out in a reactive distillation column in which the catalyst is flooded. Although the reaction to form MTBE can proceed more readily in a liquid phase, the distillation is made more difficult, as a result of which the separation of the components to produce highly pure products is not ensured.
EP 0 885 866 A1 discloses a process in 6 embodiments for preparing MTBE and a low-isobutene C4 stream by reacting a C4-hydrocarbon stream containing isobutene and n-butenes with methanol. The feature common to all embodiments is that at least one prereactor, a reactive distillation column and an after-reactor are connected in series.
In all three abovementioned publications, neither the quality of the MTBE prepared nor the isobutene content of the remaining C4 stream is disclosed.
U.S. Pat. No. 5,368,691 describes the reaction of a C4-hydrocarbon mixture containing isobutene and linear butenes with methanol to form MTBE and a C4 stream containing the linear butenes in a reactive distillation column. Here, MTBE is obtained as bottom product in a purity of greater than 98%, which does not meet the requirements for the preparation of solvent-quality MTBE. The example describes a top product having a residual isobutene content of 1.4%. This isobutene content is far too high for further processing to produce pure 1-butene. The reflux ratio of the column is stated to be from 0.5:1 to 5:1.
A further process for preparing MTBE and a low-butene C4 stream using a reactive distillation column is described in U.S. Pat. No. 4,475,005. Here, the column is operated at a reflux ratio of 1. The isobutene content of the distillate is 4830 ppm by weight and is thus significantly too high for further use for producing pure 1-butene.
Since the known processes are not fully satisfactory in respect of the isobutene content of the top product or of the raffinate II produced therefrom and/or the quality of the MTBE obtained and/or capital cost and/or energy consumption, it is desirable to develop a process which produces a raffinate II which is suitable for cost-effective production of 1-butene and at the same time gives MTBE in solvent quality.
One object of the invention is to develop a process which produces a raffinate II which is suitable for cost-effective production of 1-butene.
Another object of the invention is to develop a process which produces a raffinate II which is suitable for cost-effective production of 1-butene and at the same time gives MTBE in solvent quality.
It has surprisingly been found that the acid-catalyzed reaction of methanol and a C4-olefin mixture in a two-stage plant with a reactive distillation column as second stage makes it possible to obtain an overall isobutene conversion of above 99.9% and a virtually isobutene-free raffinate II and at the same time gives an MTBE which contains virtually no impurities if specific reaction conditions in terms of reflux ratio, temperature and pressure are adhered to in the reactive distillation column.
Thus, one embodiment of the present invention provides a process, which includes:
contacting at least one C4-hydrocarbon stream including isobutene with methanol over at least one acid catalyst and preparing methyl tert-butyl ether (MTBE) and a substantially isobutene-free C4-hydrocarbon mixture; wherein
in a first stage in one or more first reactors, the isobutene reacts with the methanol over an acid catalyst to form an equilibrium mixture including MTBE, remaining isobutene and methanol;
the equilibrium mixture is fed to a second stage including a reactive distillation column; and
in the reactive distillation column, the remaining isobutene reacts with methanol over an acid ion exchange resin to form MTBE;
wherein the reactive distillation column is operated at a pressure ranging from 3 to 15 bar abs., a reaction zone temperature ranging from 55xc2x0 C. to 75xc2x0 C., and a reflux ratio of less than 1.
Another embodiment of the present invention provides a method of making gasoline, which includes the above process and contacting the MTBE with a fuel, to obtain gasoline.